Partially amorphous nickel-iron layered double hydroxide nanosheet arrays for robust bifunctional electrocatalysis

Article abstract of DOI:10.1039/c8ta05054f

Bifunctional electrocatalysts that can boost energy-related reactions are urgently in demand for pursual of dual and even multiple targets towards practical applications such as energy conversion, clean fuel production and pollution treatment. Herein, we highlight that an in situ grown nickel-iron layered double hydroxide (NiFe LDH) nanosheet array catalyst with partially amorphous characteristics, rich native Ni³⁺ ions and an optimal Ni:Fe ratio can exhibit robust performances on both the oxygen evolution reaction (OER) and the urea oxidation reaction (UOR). Benefitting from the partially amorphous feature, the catalytically active high-valence species are easy to generate and stabilize, thus further realizing enhanced electrooxidation activity with the aid of an internal 2D charge transfer pathway and native Ni³⁺ ions. As expected, the partially amorphous catalyst exhibits a higher OER current of 284.4 mA cm^-2 at an overpotential of 500 mV, which shows 2.2-10.0 times enhancement than the counterparts with various Ni:Fe ratios. In addition, the UOR current density of the partially amorphous catalyst at 1.8 V vs. RHE shows 1.6 and 2.4 times increment relative to fully amorphous and highly crystalline catalysts, and 2.7-9.4 fold larger than the catalysts with other Ni:Fe ratios. The optimization strategy of designing the partially amorphous bifunctional catalyst in this work may broaden the way of searching for advanced electrocatalysts for simultaneous waste water treatment and clean energy production.

Full text of DOI:10.1039/c8ta05054f

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Partially Amorphous Nickel-Iron Layered Double Hydroxide
Nanosheet Arrays for Robust Bifunctional Electrocatalysis
Junfeng Xie,a,†,* Haichao Qu,a,† Fengcai Lei,a Xu Peng,b Weiwei Liu,a Li Gao,a Pin Hao,a Guanwei Cui,a
Bo Tanga,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Bifunctional electrocatalysts that can boost energy-related reactions are urgently demanded to pursue dual and even
multiple targets towards practical applications such as energy conversion, clean fuel production and pollution treatment.
Herein, we highlight that in-situ grown nickel-iron layered double hydroxide (NiFe LDH) nanosheet array catalyst with
partially amorphous characteristics, rich native Ni3+ ions and optimal Ni:Fe ratio can exhibit robust performances on both
oxygen evolution reaction (OER) and urea oxidation reaction (UOR). Benefited from the partially amorphous feature, the
catalytically active high-valence species are easy to be generated and stabilized, thus further realizing enhanced
electrooxidation activity with the aid of the internal 2D charge transfer pathway and native Ni3+ ions. As expected, the
partially amorphous catalyst exhibits a higher OER current of 284.4 mA cm-2 at an overpotential of 500 mV, which shows
2.2~10.0 times enhancement than the counterparts with various Ni:Fe ratios. In addition, the UOR current density of the
partially amorphous catalyst at 1.8 V vs. RHE shows 1.6 and 2.4 times increment relative to fully amorphous and highly
crystalline catalysts, and 2.7-9.4 folds larger than the catalysts with other Ni:Fe ratios. The optimization strategy of
designing partially amorphous bifunctional catalyst in this work may broaden the way on searching for advanced
electrocatalysts for simultaneous waste water treatment and clean energy production.
www.rsc.org/
4OH− → O2 + 2H2O + 4e−
(1)
On the other hand, urea, as a common waste in polluted water,
has been considered to be a promising chemical for hydrogen
generation in electrolyzer cells.5 During the electrocatalytic
urea oxidation reaction (UOR) in alkaline media, N2 and CO2
are generated from the anode, while water can be reduced at
the cathode to produce hydrogen via HER.6 Of note, the
equilibrium potential of the electrochemical UOR is only 0.37 V
vs. RHE, which is much lower than the equilibrium potential of
the OER (1.23 V vs. RHE).7 Therefore, UOR is in principle a
more efficient process for producing hydrogen via coupled
cathodic “water splitting”. Therefore, as a combined process
for both energy conversion and pollution treatment, UOR also
holds substantial research interests, while unfortunately few
catalysts are explored owing to the harsh reaction mechanism
which involves 6-electron transfer process (equation 2).5, 8-10
Of note, as the anodic reaction, either OER or UOR, can be
coupled with the cathodic hydrogen evolution reaction (HER,
equation 3) to realize the energy conversion from
electricity/chemical energy to clean hydrogen energy.10-13
Introduction
Electrocatalytic energy conversion has attracted extensive
attention owing to its multiple benefits such as high efficiency,
good reaction selectivity combined with environmentally
benign morality.1,
2
Among
a
large variety of the
electrocatalytic pathways, electrocatalytic oxygen evolution
reaction (OER) for water electrolysis and electrooxidation of
energy-containing small molecules like urea are highly
attractive. On the one hand, OER has not only been regarded
as an important process for overall water splitting and co-
catalysis in photoelectrochemical reactions but also as the
rate-limiting half reaction in water electrolysis owing to its
sluggish 4-electron reaction mechanism (equation 1):3, 4
CO(NH2)2 + 6OH− → N2 + 5H2O + CO2 + 6e−
2H2O + 2e− → H2 + 2OH−
(2)
(3)
Based on this understanding and considering the variable urea
concentration in industrial and sanitary waste water, exploring
bifunctional OER/UOR electrocatalysts is urgently demanded,
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which can be directly used in electrochemical pollution
treatment of urea-containing waste water and synergistically
realize water splitting, while unfortunately few such catalysts
are developed up to date.
Preparation of pa-NiFe LDH NS/NIF
Typically, 0.02 mmol urea was dissolved in 80 mL deionized
water and stirred for 10 min to form a clear solution. Then, a
piece of NiFe alloy foam (Ni:Fe ratio 1:5, denoted as NiFe5 alloy
foam, 1 cm×2 cm×1 mm) was treated by ultrasound in acetone
for 5 min, then dry naturally at room temperature. The alloy
foam was then immersed in 1% HCl solution, and once the
bubble appeared the foam was allowed to be washed by
deionized water immediately to ensure the removal of surface
oxide and avoid obvious corrosion of the metallic skeleton.
After that, the clean alloy foam was placed in Teflon-lined
stainless steel autoclave and the urea solution was added. The
reaction system was maintained at 100 oC for 12 hours, and
then cooled down to room temperature naturally. The as-
obtained black foam electrode was rinsed with distilled water
and ethanol for several times and finally dried in vacuum at 60
oC overnight.
Recently, oxygen-evolving electrocatalysts based on nickel-
iron compounds have been widely studied benefited from
their high Earth abundance, low cost and relatively high
activity.14-20 Both theoretical and experimental results have
confirmed that Ni:Fe ratio is crucial in regulating the OER
activity, and
a
pre-oxidation process of Ni2+ species is
considered to be the prerequisite to catalyze OER, which is
required to generate the catalytically active high-valence
materials, commonly in the form of Fe:NiOOH, where the
valence of nickel is from +3 to +4;21-23 while as another
electrooxidation reaction, the active sites of UOR are also
suggested to be located at Ni3+ ions,8 and therefore increasing
the intrinsic Ni3+ concentration or realizing facile generation of
these high-valence species is also of great importance in
optimizing the UOR activity. In addition, understanding the
role of Ni:Fe ratio in UOR catalysis is also meaningful in guiding
future designing of NiFe-based UOR catalysts. Towards the
engineering of catalytically active species, constructing
amorphous structure is usually considered to be an effective
method to increase the active site generation of OER catalysts,
in particular, NiFe-based materials,24-28 while the negative
influence in decreasing the intrinsic conductivity is worthy to
be particularly noted. In this case, modulating the crystallinity
Modulation of crystallinity
In order to obtain fully amorphous NiFeOx film on NiFe alloy
foam (a-NiFeOx/NIF) and highly crystalline NiFe LDH
nanoplates on NiFe alloy foam (c-NiFe LDH NP/NIF), the
o
o
synthesis temperatures were modulated as 80 C and 140 C,
respectively, and other operations are the same as the
synthesis process of pa-NiFe LDH NS/NIF.
Preparation of NiFe2O4 NP/NIF, Ni(OH)2/NF and γ-Fe2O3/IF
By replacing the NiFe5 alloy foam as NiFe7.7 alloy foam, pure Ni
foam, pure Fe foam and keeping other operations unchanged,
NiFe2O4 nanoplates grown on NiFe foam (NiFe2O4 NP/NIF),
Ni(OH)2 nanosheets grown on nickel foam (Ni(OH)2/NF) as well
as γ-Fe2O3 grown on iron foam (γ-Fe2O3/IF) can be obtained.
Structural characterizations
to fabricate partially amorphous structure would be
a
promising strategy in enhancing the electrooxidation
performances, which could achieve an optimal balance
between active sites and conductivity. In this work, by means
of in-situ growing nickel-iron layered double hydroxide (NiFe
LDH) nanosheet arrays on NiFe alloy foam, a self-supported
anode with partially amorphous feature is fabricated, realizing
robust performance towards OER and UOR catalysis. The
partially amorphous nanosheets can not only provide
abundant reactive sites for the electrochemical generation of
high-valence species, but also result in rich crystalline-
amorphous interfaces to form and stabilize native Ni3+ ions.
Furthermore, the highly porous and conductive alloy skeleton
accompanied by the nanosheet sub-units with high surface
area and 2D charge transfer pathway further accelerate the
electrocatalytic rate, and finally resulting in excellent OER and
UOR activities.
X-ray diffraction (XRD) was performed on a Philips X’Pert Pro
Super diffractometer with Cu Kα radiation (λ = 1.54178 Å). The
scanning electron microscopy (SEM) images were taken on a
JEOL JSM-6700F SEM. The transmission electron microscopy
(TEM) was carried out on a JEM-2100F field emission electron
microscope at an acceleration voltage of 200 kV. The high-
resolution TEM (HRTEM) was performed on a JEOL-2010
transmission electron microscope at an acceleration voltage of
200 kV. X-ray photoelectron spectroscopy (XPS) analyses were
performed on
a VGESCALAB MKII X-ray photoelectron
spectrometer with an excitation source of Mg Kα = 1253.6 eV,
and the resolution level was lower than 1 atom%. The
inductively coupled plasma (ICP) emission spectrum was
conducted on a Perkin Elmer Optima 7300DV ICP emission
spectroscope.
Experimental
Materials
Electrocatalytic study
The nickel foam, iron foam and the alloyed NiFe foams were All the electrochemical measurements were performed in a
purchased from Suzhou Jiashide Co., Ltd. and the other three-electrode system linked with an electrochemical
reagents were purchased from Sinopharm Chemical Reagent workstation (CHI660E) at room temperature. All potentials
Co., Ltd.. All chemicals were used as received unless noted were calibrated to a reversible hydrogen electrode (RHE) and
specially.
the data are presented without iR correction. In OER
measurements, cyclic voltammetry (CV) and linear sweep
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voltammetry (LSV) with a scan rate of 50 mV s-1 were obtained partially amorphous sample. As shown in Fig. 1A,
conducted in 1 M KOH aqueous solution (pH=14, saturated besides the peaks originated from the alloy substrate, three
with pure O2). A Hg/HgO electrode was used as the reference sharp peaks located at 11.2o, 22.5o and 34.3o can be revealed
electrode, a platinum gauze electrode (2 cm×2 cm, 60 mesh) with d spacings of 7.90 Å, 3.95 Å and 2.63 Å, corresponding to
was used as the counter electrode, and the treated foam the (003), (006) and (009) reflections of NiFe LDH with
electrode served as the working electrode which was fixed preferential exposure of the basal planes. In addition, an
with an electrode holder. For the chronoamperometry test, a extensively broadened peak in the range of 17o~36o can be
static overpotential of 370 mV (1.6 V vs. RHE) was applied. As observed, which is characteristic for amorphous structure,
for the UOR test, all operations were the same as above except therefore suggesting the partially amorphous nature of the
that the electrolyte was replaced by the mixed solution product.29, 30 In addition, by replacing the alloy foam to NiFe
containing 1 M KOH and 0.33 M urea. The electrochemical alloy foam with higher Fe content, pure Ni foam, and pure Fe
impedance spectroscopy (EIS) measurements were operated in foam, spinel NiFe2O4 nanoplates grown on NiFe foam (NiFe2O4
the same configuration at open circuit voltage from 10-2-105 NP/NIF), Ni(OH)2 nanosheets grown on nickel foam
Hz. The impedance data were fitted to a simplified Randles (Ni(OH)2/NF) as well as γ-Fe2O3 grown on iron foam (γ-
circuit to extract the series and charge-transfer resistances.
Fe2O3/IF) can be synthesized as counterparts with different
Ni:Fe ratios (Fig. S3-4).
Results and discussion
Structural and elemental characterizations
Scheme 1. Illustration of the formation processes of catalysts.
Fully amorphous, partially amorphous and highly crystalline
catalysts can be fabricated by tuning the synthesis
temperature.
By serving NiFe alloy foam as both metal precursor and
conductive substrate, partially amorphous nickel-iron layered
double hydroxide nanosheet arrays grown on NiFe alloy foam
(denoted as pa-NiFe LDH NS/NIF) were prepared, which can be
used directly as a freestanding anode for water electrolysis
(Scheme 1). During the formation process, urea molecules can
offer a weak alkaline environment for the oxidation of metal
substrate to form corresponding hydroxide, and the carbonate
ions from the decomposition of urea can be simultaneously
intercalated into hydroxide layers to form LDH structure. Of
note, the crystallinity of products can be rationally modulated
by simply varying the synthesis temperature. Fully amorphous
NiFeOx film on NiFe alloy foam (a-NiFeOx/NIF) and highly
crystalline NiFe LDH nanoplates on NiFe alloy foam (c-NiFe LDH
NP/NIF) can be formed (Scheme 1, Fig. S1-2) at lower and
Fig. 1 (A) XRD pattern of pa-NiFe LDH NS/NIF. (B) SEM image of
pa-NiFe LDH nanosheet arrays grown on the alloy foam. (C)
TEM image of the pa-NiFe LDH nanosheets separated from the
alloy foam. The inset shows SAED pattern. (D) HRTEM image of
pa-NiFe LDH nanosheets indicates the co-existence of
crystalline and amorphous structure. The crystalline-
amorphous interface is labeled by yellow dash line. (E-F) XPS
spectra of nickel and iron for a-NiFeOx/NIF, pa-NiFe LDH
NS/NIF and c-NiFe LDH NP/NIF.
o
higher temperatures, while at medium temperature (100 C),
NiFe LDH nanosheet arrays with partially amorphous feature
can be obtained. X-ray diffraction (XRD) technique was first
conducted to investigate the structural information of the
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In order to further study the morphological information of while the Ni3+ ions in c-NiFe LDH NP/NIF may arise from the
pa-NiFe LDH NS/NIF, scanning electron microscopy (SEM) and higher synthesis temperature that endows a higher oxidation
transmission electron microscopy (TEM) were performed. As level of metal foam, which is crucial for the stabilization of LDH
shown in Fig. 1B, well-defined ultrathin nanosheets can be structure that requires proper M2+:M3+ ratio. Besides, the Fe
observed with homogeneous lateral size of ~200 nm and 2p profile (Fig. 1F) located at 713.1 and 725.2 eV can be
thickness less than 10 nm. The thin layers may provide more assigned to Fe 2p3/2 and Fe 2p1/2 for Fe3+ species,
reactive sites for the pre-oxidation of low-valence metal ions respectively.40, 41 The Fe 2p3/2 is accompanied by a satellite line
to electrochemically active high-valence species and thus at 718.3 eV, indicating the presence of Fe3+ ions.42 In addition,
benefit the electrooxidation behavior.31-33 TEM image in Fig. 1C the peak at 714.1 eV implies that Fe3+ ions exist in more than
further confirms the ultrathin nanosheet morphology with one coordination environment, which could be attributed to
hierarchical porous structure, which is beneficial for the the high affinity of neighboring Ni2+ ions.42,
43
Therefore,
diffusion of electrolyte as well as the gaseous or ionic Ni5.75Fe LDH with partially oxidized Ni3+ ions can be confirmed.
products.34, The selected area electron diffraction (SAED) The native Ni3+ ions may play a key role in stabilizing the LDH
35
pattern in the inset of Fig. 1C further demonstrates the structure to achieve both optimal Fe concentration and
partially amorphous feature of the product, where a relatively M2+:M3+ ratio, and offer
a
potential in boosting the
bright halo around the center spot can be identified, which can electrocatalytic oxidation reactions such as OER and UOR.
be considered as a solid evidence for the presence of
amorphous species. Of note, considering the relatively low
concentration of the amorphous species in the partially
amorphous NiFe LDH nanosheet, the halo is not as bright as
the typical fully amorphous material. In order to further
understand the partially amorphous structure, high-resolution
TEM (HRTEM) was carried out. As shown in Fig. 1D, interplanar
spacing of 2.40 Å can be indexed, matching well with the (012)
facets of NiFe LDH.36 Of note, typical amorphous species with
no crystal fringes can be revealed at the edge region of the
nanosheets, therefore proving the partially amorphous
structure of the as-grown nanosheets.
Electrochemical impedance spectroscopy and electrochemical
double-layer capacitance
Electrochemical impedance spectroscopy (EIS) was performed
to investigate the electrochemical kinetics and conductivity of
different catalysts. As can be seen in Fig. 2A and Table 1, all
catalysts display similar small series resistances of ~3 Ω at 0.9
V vs. RHE, which are benefited from the highly conductive
metal foam skeleton. Of note, the much smaller radius of
curvature for pa-NiFe LDH NS/NIF suggests the smaller charge
transfer resistance (Rct), which could be fitted to be 1236 Ω at
a non-redox voltage of 0.9 V vs. RHE, much smaller than its
counterparts. The smaller Rct value may arise from the 2D
conductive pathway along the vertically aligned ultrathin
nanosheets.12, 44 Of note, the smaller Rct value of the partially
amorphous catalyst may boost the pre-oxidation process to
reach a more facile generation of the catalytically active high-
valence species, thereby lead to better performance for
electrooxidation reactions.
X-ray photoelectron spectroscopy (XPS) was performed to
survey the composition and valence information of pa-NiFe
LDH NS/NIF and its fully amorphous and highly crystalline
counterparts. Since XPS technique is highly surface sensitive,
the as-measured Ni:Fe atomic ratio of 5.75:1 can be regarded
as the composition of partially amorphous NiFe LDH
nanosheets. Of note, the Fe concentration of 14.8% is right in
the range of broad maximum in OER activity according to
previous reports,14 suggesting its high potential for OER. As can
be seen from Fig. 1E, the Ni 2p spectrum of pa-NiFe LDH
NS/NIF can be fitted as two spin-orbit doublets, characteristic
of Ni2+ and Ni3+, and two shakeup satellites.37, 38 The binding
energy of Ni 2p3/2 region can be deconvoluted into two peaks
at 855.5 eV and 857.1 eV that match the oxidation states of
Ni2+ and Ni3+, respectively; while the binding energy of Ni 2p1/2
region can be deconvoluted into two peaks centered at 873.3
eV and 874.7 eV, corresponding to Ni2+ and Ni3+, respectively.37
Furthermore, two broad peaks at 861.8 eV and 879.8 eV can
be indexed to the satellite peaks of 2p3/2 and 2p1/2 spin orbits,
respectively. Therefore, the mixed valence of nickel can be
confirmed, and the presence of native Ni3+ species may
originate from the abundant crystalline/amorphous interfaces
as well as the amorphous structure where the coordinating
environment of nickel is strongly varied.39 Similarly, native Ni3+
ions can also be detected for a-NiFeOx/NIF owing to its fully
amorphous structure with abundant low-coordinated Ni ions,
Fig. 2 (A) Nyquist plots of catalysts with different Ni:Fe ratios
and crystallinity at 0.9 V vs. RHE. Inset shows the equivalent
circuit. (B) The current density variation at a non-redox region
plotted against scan rates to estimate Cdl.
Besides the conductivity issues, electrochemical surface
area (ECSA) is another key parameter that strongly influences
the electrocatalytic behaviors. Electrochemical double-layer
capacitance (Cdl), which is linearly proportional to ECSA, is
well-accepted to be an effective criterion to evaluate the ECSA
of electrocatalysts.44 As shown in Fig. 2B and Table 1, the Cdl
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value of pa-NiFe LDH NS/NIF is calculated to be 2.2 mF cm-2, provide more reactive surface sites and fast charge transport.
which is similar to other counterparts with those values in the The easier oxidation can lead to faster generation of
range of 1.23-1.98 mF cm-2. The similarly high Cdl values arise catalytically active Ni3+ species at a relatively low bias, thus
from the highly porous metal foam skeleton with surface accelerating the electrooxidation reactions such as OER and
nanostructures that guarantees large surface area to adsorb UOR. In addition, when applying potentials beyond the
ions in the electrolyte. Since Cdl is proportional to ECSA, the oxidation peak, the anodic current density of pa-NiFe LDH
structural benefits of metal foam-based catalysts can be NS/NIF increases dramatically and achieves a 30 mA cm-2
verified, which is advantageous to electrocatalytic processes current at an overpotential (η30) of only 233 mV. While for
such as OER and UOR.
Table 1 Comparison of series resistances, charge transfer higher overpotentials of 273-505 mV are required to reach the
resistances and electrochemical double-layer capacitances.
other catalysts with various crystallinity and Ni:Fe ratio, much
same current density (Table 2), thus confirming the high OER
activity of pa-NiFe LDH NS/NIF. Of note, this value (η30=233
mV) is smaller than that of the NiFe LDH/Ni foam from solution
precursor (η30=280 mV), which may arise from the ultimate
attachment between the in-situ formed NiFe LDH nanosheets
and the alloy substrate.46 Furthermore, to achieve a higher
current of 100 mA cm-2, pa-NiFe LDH NS/NIF only requires a
low η100 of 326 mV, which is 40-201 mV lower than the other
counterparts (Table 2), thus further demonstrating its high
OER activity. Moreover, when applying higher overpotential,
the OER activity of pa-NiFe LDH NS/NIF is even more
predominant. For instance, at η=500 mV, pa-NiFe LDH NS/NIF
possesses a high anodic current density of 284.4 mA cm-2,
which shows 2.2-10.0 times enhancement than other catalysts
with various Ni:Fe ratios. Of note, benefited from the highly
exposed low-coordinated Ni ions, fully amorphous a-
NiFeOx/NIF also exhibits good OER activity with jgeo of 249.5
mA cm-2 at η=500 mV, while its drawback in electrochemical
stability will be discussed later. In addition, in order to
investigate the intrinsic OER activity, the LSV curves were
normalized with Cdl values to exclude the influence of surface
Rs [Ω]
2.9
Rct [Ω] Cdl [mF cm-2]
pa-NiFe LDH NS/NIF
a-NiFeOx/NIF
1236
2235
2018
2875
3326
3020
2.02
1.98
1.62
1.78
1.55
1.23
3.3
c-NiFe LDH NP/NIF
NiFe2O4 NP/NIF
Ni(OH)2/NF
2.9
3.2
3.7
γ-Fe2O3/IF
3.1
Investigations on oxygen evolution reaction
In order to verify the influences of crystallinity and Ni:Fe ratio
to the electrocatalytic behaviors, oxygen-evolving activity was
firstly investigated for these freestanding electrodes in O2-
saturated 1 M KOH solution. As can be seen from the
polarization curves in Fig. 3A, pa-NiFe LDH NS/NIF shows an
earlier oxidation peak centered at ~1.39
V vs. RHE,
corresponding to the electrochemical oxidation from divalent
Ni ions to catalytically active Ni3+ species (equation 4). Of note,
this oxidation peak is approximately 20-80 mV lower than
those of Ni-containing counterparts, suggesting the easier
electrooxidation of Ni2+ in pa-NiFe LDH NS/NIF. The possible
reason for the negative shift in Ni2+-to-Ni3+ oxidation peak may
arise from the partially amorphous structure which offers both
electrochemically active low-coordinated Ni ions in amorphous
area and facile charge transport guaranteed by the crystalline
basal planes. Interestingly, the peak potential of pa-NiFe LDH
NS/NIF matches well with the former result on highly porous
β-Ni(OH)2 nanomesh with rich coordinately unsaturated Ni
ions, indicating the similar oxidation kinetics during positive
sweep.45 Besides, no oxidation peak can be observed for γ-
Fe2O3/IF, which is consistent with previous reports for iron
oxide based OER catalysts.40 Accordingly, Tafel plots in Region
1 (Fig. 3B) corresponds to the Ni2+-to-Ni3+ oxidation process,
and the fitted Tafel slopes of Ni-containing catalysts can be
used to estimate the ease of occurrence for the following
reaction:
area (Fig. S8). As listed in Table 2, the normalized current (jCdl
)
of pa-NiFe LDH NS/NIF at η=300 mV reaches 37.2 A F-1, which
is roughly 1.5-28.6 times larger than its counterparts. Besides,
the identification of onset overpotential is difficult since the
starting region of OER is overlapped by the oxidation peak,
while considering the decreasing tail of this oxidation peak, the
onset overpotential should be lower than 210 mV.45 From the
cathodic line of CV curve (Fig. S10A), the onset overpotential is
estimated to be as low as 175 mV, confirming the superior
activity of pa-NiFe LDH NS/NIF.47 Same interferences can also
be observed in corresponding Tafel plots. As shown in Fig. 3B
and Table 2, the Tafel slopes in Region 2 of all Ni-containing
catalysts are rather large, with value of 157 mV decade-1 for
pa-NiFe LDH NS/NIF and even larger for other Ni-containing
counterparts (176-445 mV decade-1), which can be attributed
to the severe interference of the Ni2+-to-Ni3+ oxidation peak to
the OER onset region.45 In sharp contrast, γ-Fe2O3/IF without
such an ion oxidation process exhibits smaller Tafel slope of
107 mV decade-1. In order to exclude this interference, Tafel
curve from the cathodic sweep of CV curve was plotted,47 from
which a much smaller Tafel slope of 63 mV decade-1 can be
identified for pa-NiFe LDH NS/NIF (Fig. S10B), thus confirming
its facile OER kinetics.
Ni2+ + 3OH− → NiOOH + H2O + e−
(4)
Focused on the Ni2+-to-Ni3+ oxidation process in Region 1, the
Tafel slope of pa-NiFe LDH NS/NIF is measured to be 38 mV
decade-1, which is smaller than the other Ni-containing
counterparts with values in the range of 50-79 mV decade-1
(Fig. 3B and Table 2), thus confirming the more facile reaction
kinetics from catalytically inert Ni2+ species to active Ni3+ ions.
The smaller Tafel slope of pa-NiFe LDH NS/NIF may arise from
the abundant crystalline/amorphous interfaces that could
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Table 2 Comparison of electrochemical parameters of OER.
jgeo
(η=500 mV)
[mA cm-2]
284.4
jCdl
(η=300 mV)
[A F-1]
37.2
Tafel slope
Tafel slope
(Region2)
[mV decade-1]
157
η30mA cm-2 η100mA cm-2
(Region1)
[mV decade-1]
[mV]
[mV]
pa-NiFe LDH NS/NIF
a-NiFeOx/NIF
233
283
273
307
505
443
326
366
406
453
-
38
64
79
50
57
-
249.5
20.3
176
c-NiFe LDH NP/NIF
NiFe2O4 NP/NIF
Ni(OH)2/NF
160.8
24.5
178
128.9
14.9
197
28.6
11.6
445
γ-Fe2O3/IF
527
75.1
1.3
107
3C, slight degradation of current density can be revealed
during CV cycling, and as the cycling proceeds, the current
decreased at a slower rate, for instance, the curves after 3000
and 4000 cycles are almost unchanged. After 4000 CV cycles, a
large OER current of 239 mA cm-2 can still be obtained at
η=500 mV, indicating the high activity during long-term CV
cycling. The operational stability was further evaluated by
chronoamperometry test under fixed overpotential. As shown
in Fig. 3D, the current density at η=370 mV only shows a slight
degradation of ~5.3% after 5-hour continuous OER operation.
The current retention of 94.7% of partially amorphous catalyst
is similar to that of the highly crystalline sample (93.8%) and
much higher than the fully amorphous catalyst (73.5%) which
is considered to be relatively soluble under continuous
operation (Fig. S11).48-50 Furthermore, as confirmed by SEM
image, the nanosheet morphology is retained after long-term
OER operation (inset of Fig. 3D), while the XPS analyses of the
pa-NiFe LDH NS/NIF catalyst after 10 h continuous OER
operation indicate that the proportion of high-valence Ni and
Fe species increases compared with the fresh catalyst (Fig.
S12), which can be attributed to the surface activation of the
catalyst.
Fig. 3 (A) Linear sweep voltammetry (LSV) curves measured in
1 M KOH solution. (B) Corresponding Tafel plots of various
catalysts. The light green shadow highlights the Tafel region
for Ni2+-to-Ni3+ oxidation, while the Tafel curves in the orange
region represent a combined process of OER and Ni2+-to-Ni3+
oxidation (γ-Fe2O3/IF for OER only). (C) Stability test of pa-NiFe
LDH NS/NIF for long-term CV cycling. (D) Chronoamperometry
data of pa-NiFe LDH NS/NIF obtained in 1 M KOH solution at
an overpotential of 370 mV. The inset shows the SEM image
after OER chronoamperometry test.
Investigations on urea oxidation reaction
Beyond OER, electrocatalytic urea oxidation reaction is also of
great importance as a combined process to synergistically
achieve both energy conversion and waste water treatment.
According to previous report, the UOR process (equation 2) is a
combined process that involves an electrochemical oxidation
step (equation 5) and a chemical redox step (equation 6),51 as
shown below:
Apart from the OER activity, the electrochemical stability is
also important to evaluate an advanced electrocatalyst,
especially for practical applications. Long-term cyclic
voltammetry (CV) and chronoamperometry were performed to
describe the stability of pa-NiFe LDH NS/NIF. As shown in Fig.
Electrochemical process:
6Fe:Ni(OH)2 + 6OH− ↔ 6Fe:NiOOH + 6H2O + 6e−
(5)
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Chemical process:
By using the optimized electrolyte with 1 M KOH and 0.33
M urea, UOR curves of various catalysts can be obtained. As
shown in Fig. 4A-B, pa-NiFe LDH NS/NIF demonstrates
excellent performance with a high UOR catalytic activity and
favorable reaction kinetics. The UOR onset potential can be
identified to be 1.322 V vs. RHE, which is close to the onset of
the Ni2+-to-Ni3+ oxidation in OER curve (Fig. S14). The early
UOR onset may arise from the native Ni3+ active sites that can
trigger UOR at a relatively low bias. Besides, geometric current
density is another key parameter to evaluate the UOR activity.
As shown in Fig. 4A and Table 3, to reach a 30 mA cm-2 current,
pa-NiFe LDH NS/NIF only requires a low bias of 1.362 V vs.
RHE, which is 30 mV and 43 mV lower than the fully
amorphous and highly crystalline catalysts with the same Ni:Fe
ratio. When compared with the catalysts with different Ni:Fe
ratios, the UOR activity of pa-NiFe LDH NS/NIF is more
prominent (Table 3), and much higher potentials of 1.471-
1.749 mV vs. RHE are needed to reach the same current
density. In addition, this superiority is even larger when
pursuing a higher current. For instance, an applied potential of
1.459 V vs. RHE can lead to a high current density of 100 mA
cm-2 for pa-NiFe LDH NS/NIF, while for fully amorphous and
highly crystalline ones, 106 mV and 174 mV higher potentials
are required, respectively. Besides, at an applied potential of
1.8 V vs. RHE, pa-NiFe LDH NS/NIF with partially amorphous
feature shows a much larger UOR current of 440.6 mA cm-2,
which is roughly 1.6 and 2.4 times larger than the fully
amorphous and highly crystalline catalysts, and 2.7-9.4 folds
larger than the catalysts with other Ni:Fe ratios. The enhanced
UOR activity of pa-NiFe LDH NS/NIF may arise from the easier
generation of high-valence species as well as the native Ni3+
ions that can facilitate the electrochemical adsorption of urea
molecules and subsequent raise the UOR rate. Similar to OER
analysis, UOR currents normalized by Cdl were also provided to
eliminate the influence brought by variable ECSA. As shown in
Fig. S15 and Table 3, when applying a bias of 1.8 V vs. RHE, a
high normalized current of 221.2 A F-1 can be obtained for pa-
NiFe LDH NS/NIF, which is much larger than the other catalysts
(35.8-139.3 A F-1) with either different crystallinity or Ni:Fe
ratio, therefore confirming the high activity in UOR catalysis.
The facile reaction kinetics can be further verified by analyzing
the corresponding Tafel plots. As shown in Fig. 4B and Table 3,
pa-NiFe LDH NS/NIF demonstrates the smallest Tafel slope of
33 mV decade-1, which can be attributed to its nanosheet array
morphology with good in-plane conductivity and rich active
sites. c-NiFe LDH NP/NIF and Ni(OH)2/NF with similar
morphology also show relatively low Tafel slope of 39 and 38
mV decade-1, respectively, while in this case the low density of
active sites restricts their UOR activity. For the other
counterparts with poor crystallinity or non-sheet surface
morphology, the Tafel slopes are much larger, thus severely
restricting their UOR behavior. A comparison of UOR activity
between pa-NiFe LDH NS/NIF catalysts and other as-explored
UOR catalysts is summarized in Fig. 4C,53-57 from which the
6Fe:NiOOH + CO(NH2)2 + H2O
→ 6Fe:Ni(OH)2 + N2 + CO2 (6)
where the NiFe LDH is denoted as Fe:Ni(OH)2, and Fe:NiOOH
represents its high-valence state after electrooxidation. As
demonstrated in previous research, the UOR process can be
divided into two individual steps, that is, i) electrochemical
oxidation process of Ni-containing catalyst to generate high-
valence phase; and ii) chemical oxidation of urea molecules on
surface of as-generated high-valence oxidative catalyst. Herein,
the native Ni3+ ions in the partially amorphous nanosheet may
trigger UOR process at a lower bias, and the amorphous edges
with low-coordinated Ni ions can be easily oxidized into high-
valence phases and thus further improving the UOR activity. In
order to achieve a good UOR performance, LSV curves were
measured in 1 M KOH solution containing various urea
concentration for pa-NiFe LDH NS/NIF. As can be seen from Fig.
S13, the optimal urea concentration to achieve a higher UOR
current can be determined to be 0.33 M, which is similar to
the result in a recent work on α-Ni(OH)2/NF UOR catalyst.52
That is, at lower urea concentrations below 0.33 M, the UOR
current increases which can be attributed to the diffusion-
controlled reaction mechanism, while at concentrations higher
than 0.33 M, the UOR current decreases since most active sites
on the surface of Fe:NiOOH are covered with urea and its
oxidation intermediate molecules which in turn decreases the
oxidation rate of urea owing to the local deprivation of OH−
species.8, 52
Fig. 4 (A) UOR curves of various catalysts in 1 M KOH with 0.33
M urea. (B) Corresponding Tafel plots. (C) Comparison of UOR
activity of various catalysts measured in 1 M KOH and 0.33 M
urea. The samples "this work-1/2" represent the fresh and
activated (after 2000 CV cycles) pa-NiFe LDH NS/NIF catalysts,
respectively. (D) An activation process can be revealed along
with CV cycling test.
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dominance of UOR activity in both onset overpotential and operation. The beneficial activation effect can be further
current density under fixed potential can be clearly identified. confirmed by the variation of required potentials to achieve a
Long-term CV measurement was conducted to further UOR current density of 100 mA cm-2. For the fresh pa-NiFe LDH
evaluate the electrochemical stability of pa-NiFe LDH NS/NIF in NS/NIF catalyst, a potential of 1.459 V vs. RHE is required,
UOR catalysis. As shown in Fig. 4D, an obvious activation while along with the CV-induced activation, the demanded
process can be revealed during the first 2000 CV cycles, where bias decreases obviously and finally can be stabilized to 1.404
the peak current shows remarkable increment from 109.8 mA V vs. RHE after 2000 CV cycles. Furthermore, no current
cm-2 for the initial cycle to 264.5 mA cm-2 for the 2000th cycle. degradation appears at higher potentials, i.e. 1.8 V vs. RHE
Beyond 2000 CV cycles, the peak current only shows slight even after 2500 CV cycles, further confirming the excellent
degradation to 256.2 mA cm-2 after 2500 CV cycles, electrochemical stability of the partially amorphous
demonstrating good activity and stability for long-term UOR freestanding catalyst.
Table 3 Comparison of electrochemical parameters of UOR.
Required
potential for
30 mA cm-2
[mV vs. RHE]
1.362
Required
potential for
100 mA cm-2
[mV vs. RHE]
1.459
jgeo
jCdl
Tafel slope
(UOR)
@1.8 V vs. RHE
[mA cm-2]
@1.8 V vs. RHE
[A F-1]
[mV decade-1]
pa-NiFe LDH NS/NIF
a-NiFeOx/NIF
440.6
281.0
183.2
164.3
108.0
46.9
221.2
139.3
111.8
91.8
33
67
1.392
1.565
c-NiFe LDH NP/NIF
NiFe2O4 NP/NIF
Ni(OH)2/NF
1.405
1.633
39
1.471
1.664
61
1.492
1.744
69.1
38
γ-Fe2O3/IF
1.749
—
35.8
184
Conclusions
In summary, partially amorphous NiFe LDH nanosheet arrays
with native Ni3+ ions and optimal Ni:Fe ratio were developed,
realizing robust performance on both electrocatalytic oxygen
evolution and urea oxidation. The partially amorphous
structure can not only bring in native Ni3+ ions and lead to
facile generation of high-valence active species for these
electrooxidation reactions, but also offer 2D charge transfer
pathway to facilitate the electrocatalytic process. Besides, the
optimized Ni:Fe ratio as well as the large active surface area
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Graphical Abstract